1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal lens electrically driven and stereoscopy display device using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for reducing a thickness of a liquid crystal layer provided to the liquid crystal lens electrically driven in a manner of applying Fresnel lens within a pitch anisotropically.
2. Discussion of the Related Art
Generally, services for high speed information transfer, which will be established based on very high speed information communication network, develop into ‘viewing and listening’ multimedia type services with a digital terminal capable of high speed processing of text, speech and video from ‘listening and speaking’ services such as a phone service and are expected to eventually develop into hyper-space type real 3D stereoscopic information communication services for ‘viewing, feeling and enjoying stereoscopic reality by transcending time and space’.
A stereoscopic image for representing 3 dimensions is achieved according to the principle of stereo vision via both eyes in general. Since there exists a binocular parallax (i.e., both eyes are spaced apart from each other by about 65 mm), right and left eyes view slightly different images due to the position difference between both eyes. Thus, the image difference attributed to the position difference between both eyes is called binocular disparity. And, the 3D stereoscopic video display device uses the binocular disparity to enable left and right eyes to just see images for left and right eyes, respectively.
In particular, left and right eyes see different 2D images, respectively. The two images are delivered to a brain via retinas. The brain synthesizes the delivered images together to reproduce depth and reality of the original 3D image. This capability is called stereography and a device applying the stereography thereto is called a stereoscopy display device.
Meanwhile, the stereoscopy display device can be classified according to components that construct lens for 3D (3-dimension) implementation. For instance, a type of constructing a lens using a liquid crystal layer is called a liquid crystal field lens type.
A liquid crystal display device normally consists of two electrodes and a liquid crystal layer provided between the two electrodes. An electric field generated from applying a voltage to the two electrodes drives liquid crystal molecules of the liquid crystal layer. The liquid crystal molecules have polarization and optical anisotropy. In this case, the polarization indicates that molecular arrangement direction is changed according to an electric field and the electric charges of the liquid crystal molecules are attracted to both sides of the liquid crystal molecules when the liquid crystal molecules exist within the electric field. And, the optical anisotropy indicates a path or polarized state of a projected light is changed according to an incident direction or polarized state of an incident light due to a thin and long configuration of liquid crystal molecules and the aforesaid molecular arrangement direction.
Accordingly, the liquid crystal layer shows a transmittance difference by a voltage applied to two electrodes and is then able to display an image by differentiating the difference per pixel.
Recently, a liquid crystal lens electrically driven is proposed to enable a liquid crystal layer to play a role as a lens using the property of the liquid crystals.
In particular, a lens controls a path of an incident ray per position using a refractive index difference between air and a substance constructing the lens. If a liquid crystal layer is driven by forming an electric field by applying a different voltage per position of electrode in the liquid crystal layer, an incident ray entering the liquid crystal layer has a different phase change per position. Therefore, the liquid crystal layer is able to control the path of the incident ray like a real lens.
An electrically-driven liquid crystal lens according to a related art is explained with reference to the accompany drawings as follows.
FIG. 1 is a cross-sectional diagram of an electrically-driven liquid crystal lens according to a related art. FIG. 2 is a graph of a potential distribution after voltage application in fabricating an electrically-driven liquid crystal lens.
Referring to FIG. 1, an electrically-driven liquid crystal lens according to a related art consists of a first substrate 10, a second substrate 20 and a liquid crystal layer 30 provided between the first and second substrates 10 and 20.
In this case, a plurality of first electrodes 11 are formed on the first substrate 10 by being spaced apart from each other with a first spaced distance. Regarding the first electrodes 11 adjacent to each other, a distance between a center of one of the adjacent first electrodes 11 and the other is called a pitch. And, the identical patterns (first electrodes) are repeatedly formed using the pitch as a period.
And, a second electrode 21 is formed on the second substrate 20 configured to oppose the first substrate 10.
The first and second electrodes 11 and 21 are formed of transparent metal. The liquid crystal layer 30 is formed in the space between the first and second electrodes 11 and 21. Liquid crystals forming the liquid crystal layer 30 have a parabolic potential surface due to a characteristic reactive with strength and distribution of an electric field and corresponding have the phase distribution similar to that of an electrically-driven liquid crystal lens shown in FIG. 2.
The above-described electrically-driven liquid crystal lens is formed on the condition that a high voltage is applied to the first electrode 11 while the second electrode 21 is grounded. According to this voltage condition, a strongest vertical electric field is formed at the center of the first electrode 11. The vertical electric field becomes weaker if getting farther away from the first electrode 11. When the liquid crystal molecules constructing the liquid crystal layer 30 have anisotropy of positive dielectric constant, the liquid crystal molecules are arranged along an electric field. Therefore, the liquid crystal molecules stand vertical at the center of the first electrode 11. If the liquid crystal molecules are located farther from the first electrode 11, they tend to have arrangement inclining almost horizontally. In respect of light transfer, referring to FIG. 2, a light path at the center of the first electrode is short. If a position of the liquid crystal molecules becomes farther from the first electrode 11, the light path gets longer. If this light path is represented on a phase plane, it provides a light transfer effect similar to that of a lens having a parabolic surface.
In this case, the second electrode 21 causes fluctuation of a liquid crystal electric field to induce a refractive index of light into a spatially parabolic function. And, the first electrode 11 forms an edge area of the lens.
In doing so, the voltage applied to the first electrode 11 is slightly higher than that applied to the second electrode 21, whereby a potential difference is generated between the first and second electrodes 11 and 21, as shown in FIG. 2. Specifically, an abrupt lateral electric field is induced at a part of the first electrode 11. Consequently, the liquid crystals fail to form a smooth distribution but form a slightly distorted distribution, whereby spatial refractive index distribution is not parabolic or becomes very sensitive to a voltage.
The above electrically-driven liquid crystal lens can be fabricated in a manner of forming electrodes on both substrate by leaving the liquid crystals in-between without adopting a lens having a physically parabolic surface and then applying a voltage to the electrodes.
However, the related art electrically-driven liquid crystal lens causes the following problems.
First of all, since an electrode formed on a bottom substrate is provided to a very small portion of a lens area, an electric field between a lens edge area corresponding to the electrode and a lens center area getting farther from the lens edge area is not formed smoothly but induces an abrupt lateral electric field, whereby the electrically-driven liquid crystal lens has a slightly distorted phase. Particularly, in an electrically-driven liquid crystal lens formed by a liquid crystal electric field, if a pitch of a lens area is further increased, the electrode having a high voltage applied thereto is limited, an electric field generated between the electrode having the high voltage applied thereto in the lens area and an opposing substrate is not sufficient, it becomes more difficult to form an electrically-driven liquid crystal lens having a smoothly parabolic plane to provide the same effects of a lens.
Secondly, in a large-scale display device, since a lens center area getting distant from an edge area of a lens area, in which an electrode is located, barely has an electric field effect, it is difficult to adjust liquid crystal arrangement by an electric field in this area. Occasionally, if the adjustment in the lens center area is difficult or impossible, the corresponding electrically-driven liquid crystal lens has a discontinuous lens profile, thereby being difficult to be used as a lens.
Thirdly, a height (sag) of an electrically-driven liquid crystal lens per pitch formed according to a vertical electric field between a single electrode having a high voltage applied thereto and an electrode provided to a surface of an opposing substrate is equal and the electrically-driven liquid crystal lens should have upper and lower margin of the height. Thus, the electrically-driven liquid crystal lens needs a considerable amount of liquid crystals. As the height of the electrically-driven liquid crystal lens increases, the liquid crystal consumption increases by volume unit. Therefore, the cost is raised and the process performance is degraded, thereby requiring many efforts for enhancement.
Fourthly, a focal distance of an electrically-driven liquid crystal lens is inverse proportional to its height (sag). In order to fabricate an electrically-driven liquid crystal lens having a short focal distance, a liquid crystal layer needs a considerable height to result in a rising cost.